This invention pertains to optical filters generally, and more specifically to a class of optical elements which can be used to build optical systems with very narrow band transmission.
Narrow band optical systems have been developed for a wide variety of applications for many years. The usual objective is to isolate an optical signal of a specific wavelength in the presence of a large flux of noise, i.e., radiation at other wavelengths. The conventional systems make use of optical filters which are based on selective reflection of metals, or on dielectric layers, or on absorption. These conventional optical filters remove radiation at the unwanted wavelengths, and allow the desired wavelengths to pass through the system.
Some prior-art optical filtering systems make use of selective atomic transitions in a gaseous environment to create photons of a different wavelength from the photons of the signal wavelength, cf. J. B. Marline, J. Nilsen, L. C. West, and L. L. Wood, J. Applied Physics, V50 , No. 2, Feb. 1979. This transformation of wavelengths simplifies the process of filtering or isolating the signal photons from the noise photons.
The conventional approach to narrow band optical systems using prior art optical filters has several disadvantages. First, as the pass band is narrowed, the throughput at the desired wavelength is diminished also. Thus, low intensity radiation cannot be observed. Second, the transmission characteristics of selective reflectors is dependent on the angle which the optical rays make with the plane of the filter. This precludes the use of such filters in wide angle or fast optical systems. Third, the transmission band of conventional optical filters is affected by environmental factors such as temperature, so that the pass band may not match the wavelength of the signal.
The prior-art optical systems utilizing wavelength shifting techniques in a gaseous environment for filtering have several disadvantages. These optical systems do not allow imaging by the optical system. These optical systems are bulky because they require sufficient optical path length at low densities to efficiently convert the incoming signal to a new wavelength. Moreover, some gasses must be excited out of the ground state with a laser beam in order to obtain a fluorescent conversion that corresponds to the incoming light, cf. J. A. Gelbwachs, IEEE J. Quantum Electronics, V24, No. 7, July 1988.